By any ordinary definition, most of the moons of the solar system are "airless" -- their surfaces are surrounded by a hard vacuum. Even so, some of them do have ultra-thin "atmospheres" -- a few atoms or molecules that waft through their dark skies.

Our own Moon has an atmosphere of hydrogen, sodium, and a few other elements. They're either trapped bits of solar wind, or bits knocked off the surface by the solar wind.

Another moon with a thin atmosphere is Rhea, one of the moons of Saturn. Saturn is to the left of our Moon as they rise late this evening, and looks like a bright golden star. The true star Spica is below them; more about this lineup tomorrow.

Rhea's a ball of rock and ice that's about half the diameter of the Moon. Its surface is rugged and heavily cratered, indicating that not much happens there.

Over the last five years, the Cassini spacecraft has flown by Rhea several times, dipping quite close to its surface. During those passes, its instruments have "sniffed" traces of oxygen and carbon dioxide, showing that Rhea has a thin atmosphere.

Both probably come from the surface of Rhea itself. The oxygen probably comes from chemical reactions between the ice at Rhea's surface and particles trapped in Saturn's magnetic field. The carbon dioxide's origin is less certain, although it could come from the same process. Regardless of how it got there, though, it helps create a thin atmosphere for an icy moon.

Script by Damond Benningfield, Copyright 2010

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